A word used to describe the eligibility of a friendship or relationship based on one trait or preference of the other person. Can also be used if you don't agree with someones views.
John: "So whats your least favorite band?"
Jill: " It would have to be Arcade Fire."
John: "Wait...What? You don't like Arcade Fire? Thats a dealbreaker."
OR
Billy: "Favorite Modern Warfare 2 class?"
Johny: "Well, I usually have the UMP .45 with a Silencer, Desert Eagle with a Tac knife, Marathon Pro, Lightweight Pro, and Commando Pro.
Billy: "Woah man. Way to be a noob. We aren't friends anymore, thats a dealbreaker."
A term that's used to mean that there's something you dislike so much that could pretty much end the interaction, such as someone going off to a topic you dislike would be a dealbreaker for the chat's terms
Person1: Can't you actually just chat seriously? not everything has to be a joke.
Person2: Sorry, but that's who i am, i don't take things seriously.
Person1: Then i don't want to talk to you anymore, that's a dealbreaker for me.